Experimental Study of Solar Photovoltaic/Thermal (PV/T)
System Based on Flat Plate Heat Pipe
Zhen Hua Quan, Ning Jun Li ,Yao Hua Zhao
Architectural and Civil Engineering Institute, Beijing University of Technology, Beijing, China
quanzh@bjut.edu.cn, 81157621@163.com, yhzhao@bjut.edu.cn
Abstract: In order to improve the utilization of solar energy, we
majority of unemployed solar radiation energy is absorbed by
use the new flat-plate heat pipe in the solar photovoltaic/thermal
the cell and converted into heat energy except that the minor
(PV/T) system to meanwhile gain electricity and heat. In this way,
part is reflected. If the energy can be discharged timely and
the temperature of solar cell could be effectively reduced to
effectively utilized, energy saving will be notable. This solar
improve power generation efficiency, and the heat dissipation of
photovoltaic/thermal (PV/T) system (or solar electric and
solar panel could also be stored up through the forced
thermal combination system) has been a new concept of the
circulation of the aqueous solution at the same time for the hot
solar power generation in recent years, which has been
water supply or radiant floor heating. We had done the solar
concerned by scholars at home and abroad.
photovoltaic/thermal (PV/T) system’s experimental test at the
Research and development on solar photovoltaic/thermal at
end of heating season. The results indicated that the totally heat
home and abroad is basically that air or water is taken as
generated from a 900W solar panels could meet the heating
cooling medium for solar cell module; on such a basis, natural
2
demand of a 15m building. The instantaneous thermal efficiency
or forced circulating is employed to realize electric and
of system could reach 25.8%, the photovoltaic conversion
thermal energy combination
efficiency is nearly14.5% and the overall performance efficiency
research and development stage now and difficult to be utilized
is up to 40.3%. The solar photovoltaic/thermal (PV/T) system
in a large scale. Main reasons are their complex system, large
based on the new flat-plate heat pipe is obviously more efficient
volume, difficult integration, uneasy maintenance and high
than the photovoltaic or solar heat utilization system alone.
cost etc. Therefore, it’s necessary to seek for a reasonable, high
Keywords: photovoltaic/thermal; radiant floor heating; flat
[2-3]
. These technologies are at
efficient and low costly radiation mechanism to take heat away
and effectively utilize the heat, and enable the system under
plate heat pipe
sustainable and stable running. Professor Yaohua Zhao and
I. INTRODUCTION
Depending on the advantages, such as highly electric
others bring that the new flat heat plate is used as a heat
energy quality and cleanness, solar photovoltaic power
transfer element in heat radiation and utilization of the solar
generation has been the development direction of China and
cell module. This new flat heat pipe can solve the problem that
even world green energy. Now, important factors to affect or
multi thermal resistance contacts have to be made in normal
restrict the development of solar photovoltaic industry are low
round heat pipes, and largely improve steam heat exchange
efficiency and high cost of photovoltaic power generation.
area and overall reliability of heat pipes. It’s low costly and
From a traditional view, we only pay attention to the
has incomparable advantages from industrialized normal heat
improvement of silicon or amorphous materials to improve the
pipes. The solar photovoltaic/thermal system based on flat heat
efficiency of the solar photovoltaic power generation; while
pipe has established in laboratory of Beijing University of
the power generation efficiency of silicon batteries seriously
Technology and the system performance has been tested to
depends on the temperature actually. According to statistics,
provide theoretic basis for its application into projects.
the output power increases by 0.2-0.5% per 1k temperature
II.
PRINCIPLE AND TEST DEVICE OF PV/T SYSTEM BASED ON
[1]
decrease of the solar cell module . General commercial solar
cell has Photovoltaic conversion efficiency at 6-15%. The
FLAT PLATE HEAT PIPE
The Laying the flow channel on the back of solar cell
978-1-4244-6255-1/11/$26.00 ©2011 IEEE
module is a core of PV/T system now; while this system
300L volume and furnished with heat exchange coil pipe and
applies the high efficiency heat transfer element of new flat
1500W auxiliary electric heating rod inside. The floor
heat pipe into the solar photovoltaic/thermal system. The flat
radiation heating circulation is a circulating loop, which
plate heat pipe is made from aluminum and about 2-3mm thick
composed of water dividing and catching device, floor heating
and 28mm wide. The length depends on demands. It’s
coil pipe, water circulating pump, expansion water tank and
furnished multi independent minor heat pipes inside. The flat
heat exchange coil pipe in the hot water tank, to provide heat
plate heat pipe has the advantage of low cost, high efficiency
supply for 15m2 room. The monitoring device includes
and light and compact structure. Flat plate heat pipe,
photovoltaic controller, thermal controller, wind speed meter,
photovoltaic cell module, heat collecting water tank and
ultrasonic flow meter and thermal couple, etc.
insulation layer constitute the important components of the
The system measurement parameters include solar
radiation intensity I (W/m2), environmental temperature t s (
℃), environmental wind speed υ s (m/s), cell module current
I W (A), voltage U W (V) and surface temperature t c (℃),
heat storage water tank temperature t w (℃) and water quantity
M w (kg), indoor temperature ti ( ℃ ), floor surface
temperature t f (℃), heat collecting circulating water flow rate
mc (L/min) and heat supply circulating water flow rate
mh (L/min).
Photovoltaic conversion efficiency η e and thermal
efficiency η th are employed to evaluate the comprehensive
solar
photovoltaic/thermal
system,
namely
solar
photovoltaic/thermal modular (electricity and heat panel) and
its structure is shown in Fig. 1. The flat plate heat pipe is laid
on the back of the solar cell module. Its evaporation section
absorbs heat radiation from the cell. The condensing section of
the heat pipe is to transfer the energy to water solution in the
heat collecting water tank. Photovoltaic cell module and heat
pipe, heat pipe and heat collecting water tank are in dry contact
and silicone glue is spread inside to reduce heat transfer
resistance.
1- Glass 2-EVA 3-Silicon wafer 4- Connecting piece 5-TPT
6- Flat plate heat pipe 7- Electrode lead 8- Heat collecting
water tank 9- Insulation layer
Fig. 1 SOLAR PHOTOVOLTAIC/THERMAL MODULE
SCHEMATIC DIAGRAM
The solar photovoltaic/thermal test system is composed of
photovoltaic power generation, heat collecting circulation,
floor radiation heating circulation and monitoring devices, as
shown in Fig.2. Ten of the 90w photovoltaic cell modules
(1194mm×542mm) are employed in the system. Cell output
characteristic parameters are measured by voltmeter and
ampere meter and instantly gathered by supporting status
software. The heat collecting circulation is composed of heat
collecting water tank on the back of the electric thermal board,
connecting pipe, water pump and water tank. The water tank is
1-Photovoltaic and thermal module 2-Heat storage water tank 3-Heating rod
4-Ppump 5-Floor heating coil pip 6-Water dividing and catching device
7-Expansion water tank 8-Photovoltaic controller 9-Thermal controller
10-Storage cell 11-DC load 12-Converter 13- AC load
FIG.2 SOLAR PV/T SYSTEM SCHEMATIC DIAGRAM
performance of the PV/T system, namely total efficiency
η0 = ηe + ηth
η0 :
（1）
The Photovoltaic conversion efficiency η e of the solar
cell module is calculated by the following formula [11]:
ηe =
P U W IW
=
AI
AI
（2）
In which, P is the output power (w) of the cell module
and A is its total area (m2) including grating area.
Thermal efficiency η th is ratio between effective heat
quantity Q and total solar radiation
heat collecting:
Q0 in the period of
cell
temperature
(°C)
60
M w C p (t w 2 − t w1 )
Q
=
η th =
Q0
Q0
（3）
50
40
In which, C p is water specific heat capacity (kJ/kg·℃);
t w1 is initial water temperature in the water tank (℃); and
t w 2 is water temperature in the water tank after be heated (℃).
Total solar radiation Q0 during test period is:
30
20
10
0
8:00
outdoor
temperature
(°C)
radiation
intensity
(W/m2)
900
800
700
600
500
400
300
200
100
0
10:00 12:00 14:00 16:00 18:00
2
Q0 = A∫ Idt
（4）
FIG. 3 CURVE BETWEEN SOLAR CELL MODULE
TEMPERATURE AND TIME
1
III. TEST AND RESULT ANALYSIS
On one day of March at the end of heating period of
water tank
temperature
(°C)
2010, we conducted system performance test. We switched off
the auxiliary heating power source of the system and started
30
circulation began. The weather conditions (7:30-18:00) were
25
as follow: 14.4℃ at highest, 6.4℃ at lowest and 11.2℃ at
20
speed was 3.2m/s and the average was 1.6m/s. The strongest
2
2
solar radiation intensity was 866W/m , average was 463 W/m ,
indoor
temperature
(°C)
35
the floor radiation heating circulation as soon as heat collecting
average of outdoor temperature. The outdoor maximum wind
floor
temperature
(°C)
900
800
700
600
500
400
300
200
100
15
10
5
0
radiation
intensity
(W/m2)
8:00
10:00
12:00
14:00
16:00
18:00
2
and the total solar global radiation was 16.4 MJ/m . System
running conditions were 9.0L/min water flow rate in heat
FIG. 4 CURVE BETWEEN TEST POINT TEMPERATURE AND TIME
collecting circulation, 4.8L/min flow rate in floor radiation
heat from the floor coil pipe, and to be higher than indoor
heating circulation, initial temperature of 16.0℃ in the water
temperature so as to provide floor radiation heating. Even
tank, 12.8℃ on the floor surface and 14.8℃ in the room.
though heat collecting circulation and heat supply circulation
Fig. 3 is the curve between solar cell module temperature
were started at the same time, the water temperature in the
and time during system running. It can be seen that the
water tank reached 31.2 and the indoor temperature reached
performance of the solar cell module is directly affected by
26.3℃. It can be seen that the test system, under our test
solar radiation intensity. The temperature of the solar cell
conditions, not only improved the performance of the cell by
module had the same change trend with the solar radiation
cooling its surface temperature, but also got better heat
intensity, namely increased at first and then decreased lagging
recovery to meet the heating demand.
the solar radiation. The temperature of the cell surface was
In order to have specific photovoltaic and thermal
47.9℃ at highest and it’s obvious that the flat plate heat pipe
performance, we conducted a test without running floor
and the water circulation are contributive to solar cell cooling.
radiation heating circulation on another day in the end of
Fig. 4 is the curve between test point temperature and time. It
heating period, The weather conditions (8:30-18:00) were as
can be seen that the initial floor temperature was lower than
follow: 13.4℃ at highest, 6℃ at lowest and 8.8℃ at average
indoor temperature during the system running and the water
outdoor temperature. The outdoor maximum wind speed was
temperature in the water tank was rising during heat collecting
3.2m/s and the average was 1.6m/s. The strongest solar
process. The heat of the hot water was transferred to the floor
radiation intensity was 1038.28W/m2, the average value was
coil pipe by heat exchange pipe in the water tank. Then the
537.34 W/m2, and the total solar global radiation was 16.3
temperature of the floor surface gradually rises through getting
MJ/m2. System running conditions were 9 L/min water flow
rate in heat collecting circulation, and initial temperature of
1000
900
800
700
600
500
400
300
200
100
0
70
60
40
30
20
10
temperature
50
17:30
16:30
15:30
14:30
13:30
12:30
11:30
9:30
0
10:30
Fig.5 is the curve between solar cell module performance
and time. The solar cell module temperature rapidly rises along
with radiation intensity, as well as generation power of the cell
module, having the same changing tend with the solar radiant
intensity. Fig. 6 is the curve between heat storage water tank
temperature and thermal efficiency and time. The temperature
of hot water tank is up to 37.1℃ from 16.3℃, and the average
value is 32.8℃. The new flat plate heat pipe has non reverse
heat transfer direction, so there is no reverse heat transfer due
to temperature decrease on the solar cell module, which is the
particular characteristic of this system. In the figure, the
temperature of the heat storage water tank slowly decreases
after rising up to the highest point due to system radiation. In
addition, it can be seen from the figure that the heat rate
absorbed by the cell module is going up along with the solar
radiation intensity. After heat collection in the heat pipe, the
heat quantity taken away by the water is also going up.
Water temperature is low at the beginning, so the cell
8:30
radiation identify/module power
16.0℃ in the water tank.
radiatinal idensity （W/㎡）
cell mudule temperature ()
cell mudule power（W）
FIG.5 CURVE BETWEEN SOLAR CELL MODULAR
PERFORMANCE AND TIME
tank water temperature（℃）
photovoltaic efficiency （%）
thermal efficiency （%）
1000
900
800
700
600
500
400
300
200
100
0
thermal system has a total efficiency of more than 40% under
17:30
16:30
15:30
14:30
down gradually. It can be seen that this photovoltaic and
13:30
water temperature rises slowly, the thermal efficiency is going
12:30
8:30
highest. As the radiation intensity is going down later and the
11:30
quickly and thermal efficiency is high and up to 25.8% at
10:30
cooling effect is also good. And the water temperature rises
9:30
radiation indensity
40
35
30
25
20
15
10
5
0
temperature/efficiency
radiarion idensity （W/㎡）
FIG.6 CURVE BETWEEN SYSTEM PERFORMANCE AND TIME
non preferential weather conditioner (turning point between
(2) Under test conditions, the system effectively reduces the
winter and spring), notably improve the efficiency compared
solar cell module temperature and improves photovoltaic
with separate photovoltaic or thermal system under same
efficiency, as well as meets floor radiation heating demands.
conditions.
(3) The instantaneous thermal efficiency of system could
IV. CONCLUSION
reach 25.8%, the photovoltaic conversion efficiency is
We can get usable electricity and heat energy, as well as
nearly14.5% and the overall performance efficiency is up to
improve utility ratio of the solar energy by applying the new
40.3%. The solar photovoltaic/thermal (PV/T) system based on
flat plate heat pipe into photovoltaic and thermal system. We
the new flat-plate heat pipe is obviously more efficient than the
introduced and tested the photovoltaic and thermal system
photovoltaic or solar heat utilization system alone.
based on new flat plate heat pipe and got the conclusion as
follows:
REFERENCES
[1]
(1) The photovoltaic and thermal systems based on new
flat plate heat pipe has the particular advantages, such as
efficiency, cost and comprehensive performance, because the
flat plate heat pipe is a high efficient heat transfer element.
[2]
Wong Zhengjun, Yang Hongmei, Cooling Technologies
Applied into Condensation Solar Cell [J], Energy Technology,
2008(2): 16-18
He Wei, Chow Tin-Tai, Ji Jie, et al. Hybrid photo-voltaic and
thermal solar-collector designed for natu-ral circulation of
water [J]. Applied Energy, 2006(83): 199-220.
[3] Trpanagnostopoulos Y, et al. Hybrid photovoltaic/thermal solar
systems [J]. Solar Energy, 2002,72 (3) :217- 234.